This invention relates to semiconductor devices, and more specifically to polycrystalline silicon resistors with high, and close tolerance, resistance values.
In an integrated circuit (IC) or other device formed on a semiconductor substrate, resistors may be formed from several hundred to several thousand angstroms of thin film deposited over insulation on the surface of the substrate, and connected between two conductors. In integrated circuit devices, such isolated resistors are useful for their range of sheet resistance values, low temperature coefficients, freedom of positioning on the surface of the IC, electrical isolation from other elements of the circuit, and trimmability by a focused heat source cutting or physically evaporating a pattern of the film to increase its resistance and to improve resistance tolerance precision.
Resistive thin films are often formed of amorphous silicon, doped with chromium and traces of carbon, boron or other elements for resistance and temperature coefficient optimization. Thin film resistors of this nature are usually deposited by the well-known technique of sputtering. In sputtering, a high intensity radio frequency field is applied, through electrodes or a coil, to a low pressure gas, from which a "glow discharge" of ions bombards or "sputters" a target composed of resistor constituents in appropriate proportions, dislodging atoms from the target to condense and form a thin film on a substrate positioned nearby. Even for use as a thin film resistor, silicon must have its conductivity increased by doping, with either P type (e.g., boron) or N type (e.g., arsenic phosphorous) impurities, for hole or electron flow, respectively. Silicon can be doped with impurities while or after being deposited as a film, but whenever doping is done, the need to control doping concentration precisely makes it difficult to obtain accurate resistance values. The sputtering equipment and process are difficult to control, leaving a need for an alternate technique of providing thin film resistors.
Thin film resistors can also be formed of polycrystalline silicon deposited by conventional CVD techniques. Polycrystalline silicon is an aggregation of small, randomly oriented grains of silicon crystals whose lattices mismatch, forming boundaries between grains. Solid State Electronics, Vol. 27, No. 11, pp. 995-1001, 1984, reports that resistive thin film dopant diffusion can be controlled by combining acceptor (e.g., boron) and donor (e.g., phosphorus) doping to form a high resistance or "semi-insulating" resistor. In this prior art process, a polycrystalline silicon film is deposited on insulating oxide over a silicon substrate, blanket implanted with phosphorous atoms, annealed a first time, masked and etched, forming a pattern of polycrystalline silicon resistor areas, which are then implanted with boron atoms to a concentration no greater than that of the phosphorus atoms. The doubleimplanted film is annealed a second time, at 1000.degree. C. for at least 10 minutes, repairing implantation damage and increasing polycrystalline silicon grain size, but more importantly thermally activating dopants at an approximately predictable rate, so that practically all boron atoms become associated with, and are neutralized or deactivated by, phosphorus atoms, which offsets the conductivity, or increases the resistance, of the double-implanted resistor areas. After neutralization, resistor values are 10.sup.3 to 10.sup.4 times higher than values of conventional polycrystalline silicon thin film resistors doped with the same concentration of phosphorus. However, the resistance value tolerance remains wider than desired.